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Some thoughts from Total 911 contributor Kieron Fennelly on Porsche servicing:

It’s said that 73 percent of all Porsches ever built still exist today. While that figure might be impossible to prove, it is generally accepted that a good two thirds of the entire production is still out there.

If you own a 911, particularly an older model, the chances are you will be aware of this and, also, you will also know something about how your 911 works. Not to the extent that you would want to dismantle it yourself, perhaps, but enough to have an informed discussion with your independent Porsche specialist. 911s are reassuringly well-built in most departments, and many components on the air-cooled cars considerably were over-engineered. In return for regular maintenance and sympathetic attention the older 911 is still able to offer high performance and involving driving long after many 15 to 25 year old competitors have gone to the scrapyard.

That’s not to say that 911s don’t exhibit signs of age. They usually lose oil somewhere, although only when the level starts going down fast or there are indications that the engine is burning it is a bill – possibly large – looming. Relays fail, starter motors malfunction or seize and clutches wear. These are all difficulties that tend to have warning signs – intermittent starting problems or misfiring, for example, and could just as well plague any other car of similar age. Owning a 911 and looking after it properly is not cheap but, unlike most other top-end sports cars, the costs are generally quantifiable and major unforeseen expenditure unlikely. It’s a car which inspires confidence. After all, most 911a are still running.

There is, though, one fault which gives no forewarning at all and which can have disastrous consequences if you are very unlucky. Owners of 964s and 993s probably know that their 911s have two distributors. Previously fitted only to its racing engines, Porsche introduced the twin ignition arrangement on the M64 engine of the 964, supposedly for faster combustion and reduced emissions, and this set up was carried over to the 993. The second distributor is driven by a tiny toothed belt from the first and this is encased in the body of the unit. On the first 964s the build-up of heat and noxious oily mist and electrical arcing caused the belt to deteriorate rapidly. To solve this problem, Porsche introduced a simple ventilation hose with the 1991 model year and most existing cars had this retrofitted. Indeed, checking for the presence of the vent hose is one of the first points of advice when buying a 964.

Problem solved, then? Unfortunately, not quite. Rubber becomes brittle with age and the belt can snap like one of those red Post Office elastic bands. Sadly, belts are not usually scheduled service items and the distributor drive is no exception. On the other hand the belts for alternator or air-conditioning are visible every time you open the engine cover, so checking is easy; and if the belt on the power steering pump fails, you lose the assistance, but nothing worse.

But the distributor belt is encased and thus invisible. Even if the distributor unit is lifted from the engine, it still has to be dismantled for proper access to the belt. It is just visible through with the cap and rotor arm removed, but strictly speaking the belt should be removed from its cogs for correct examination. In a busy workshop, it’s one check that can be overlooked unless specifically requested by the 911’s owner.

That comprehensive Porsche service schedule does build a false sense of security: we tend to assume that it covers everything and that what isn’t looked at in the 12,000 miles service will be picked up at the 24,000 or 48,000 intervals. Almost, but not quite true.

If the belt does break, one rotor arm stops turning, but continues sparking. This may cause a backfire or noticeable loss of performance, thereby alerting the driver before significant damage is done if he stops in time. And more often than not, if the belt is going to fail, it does so under load on start up and should be immediately apparent. But if the rotor stops in a certain position it may cause detonations at the wrong time leading to a holed piston. One 964 owner was particularly unlucky: travelling at speed on the A1, the untimely detonation when the drivebelt broke was enough to break the crankshaft as well as igniting the unburned fuel in the exhaust, causing an engine fire. He was left with a bill for a new engine. But even a couple of damaged pistons can mean a top-end rebuild; a costly experience when not anticipated.

To put this in perspective, such breakages that have occurred seem to have afflicted mostly older 964s; 993s are younger and independent specialists have seen very few problems; although Ninemeister did have one customer whose 993 had, apparently, been running on one distributor for several years. Usually good independents will, to their credit, recommend changing the belt on any car approaching 100,000 miles or ten years and this should be the philosophy for owners. At £200-£250 for parts and labour, it’s a significant amount of money, but once it’s done you have five or ten years before you need to contemplate it again and by then you might have graduated to a 996 or 997 which, sensibly, have only one distributor.

And with that little issue out of the way, you can start worrying about another anomaly of the Porsche service schedule which on the 993 recommends oil filter changes only every second oil change; utter heresy to some people. And did you know that the 993 has a second oil filter not even mentioned in the schedule?